1. Field of the Invention.
This invention relates generally to automatic implantable defibrillators and, more particularly, to an automatic implantable defibrillator which includes a pacing, pulse generator for delivering high energy cardiac stimulating pulses to the heart after defibrillation.
2. Description of the Prior Art.
Ventricular fibrillation is a lethal cardiac arrhythmia which is characterized by disorganized electrical activity in the heart coupled with low cardiac output. The traditional treatment for ventricular fribrillation has been the delivery of a cardioverting shock to the heart to bring about the simultaneous depolarization of all of the cardiac tissue. Typically, this cardioverting shock is delivered through external paddles applied to the chest of the patient as shown in U.S. Pat. No. 3,716,059 to Welborn, et al., which discloses an external defibrillator unit coupled with a pacemaker.
An implantable form of the defibrillator is known from U.S. Pat. Nos. 3,614,954 and 3,614,955 to Mirowski, et al. Each of these devices is provided with means for detecting ventricular fibrillation and for delivering a relatively high energy cardioverting pulse (40 joules) to the heart.
Research has shown that the most effective waveform for this defibrillating or cardioverting pulse is a trapezoidal waveform. An example of a defibrillator output circuit which will deliver a trapezoidal waveshape into the heart is known from U.S. Pat. No. 3,706,313 to Milani, et al. This patent teaches the use of a current source for charging a capacitor to a high energy level which is discharged through the heart through a silicon-controlled rectifier, which is operated as a switch. A second silicon-controlled rectifier then dissipates the remaining energy on the energy storage capacitor through a resistive load, not associated with the patient's heart. This circuit provides an alternate discharge path where the energy stored in the defibillating capacitor is discharged to circuit ground.